Use the button to the right of the Function of: field to open the Independent Variable Definition editor ("Figure: Release Function Rule—Independent Variable Definition Editor"). Although limited instructions for using the Independent Variable Definition editor are provided below, the full description of how to use this editor was placed in "Commonly Used Editors and Dialogs" to minimize duplication since the Independent Variable Definition editor is used in several of places throughout ResSim.

Figure: Release Function Rule—Independent Variable Definition Editor

Release is a Function of

This field, at the top of the editor, is the independent variable (Date, Date and Time) or variable type (Model Variable, State Variable, Global Variable, or External Variable) for your function.

Variable Selection Panel

Below the Release is a Function of: field is a panel for identifying the specific variable. The variable selection panel updates depending on the release function selection.

  • If you select Date or Date and Time, no additional information is needed to define the independent variable so the Variable Selection Panel will be blank.
  • If you select Model Variable, State Variable, or Global Variable a table of available variables of that type will be displayed and you must select the specific variable for the independent variable of your function.
  • If you select External Variable, a single Variable Name field will be provided. Enter a unique name for the External Variable. Make the name descriptive enough that you will recognize it in the Alternative Editor's Time Series tab when it comes time to associate an HEC-DSS pathname to this variable.

Times Series Options

If you selected Model Variable, State Variable, Global Variable, or External Variable, the Time Series Option section is included on the right side of the Variable Selection Panel. These options allow you to define how the value of the independent variable is determined in each timestep before being used to look up the desired release from the release function.

Function

Choose a time series function from the selection list. The available functions include:

  • Previous Value — this function returns the value from the selected variable's time series that was computed at the end of the previous timestep. The previous value of several model variables, such as elevation, can be considered the starting condition of the current timestep.
  • Current Value — this function returns the value from the selected variable's time-series for the current timestep. NOTE: some variables, like inflow, are known for the current timestep while others, like elevation, have been computed yet because they are computed at the end of the timestep, after release decisions have been made.
  • Offset Value — this function returns a value from the selected time-series that is some number of hours offset from the end of the current timestep. This function requires a value in the Offset field.
  • Period Average—this function returns the average value from a set of values retrieved from two or more timesteps of the selected variable's time series. This function looks for values in both the Offset and Period fields to determine the range of values to use to compute the average.
  • Period Maximum—this function returns the maximum value from a set of values retrieved from two or more timesteps of the selected variable's time series. This function looks for values in both the Offset and Period fields to determine the range of values to use to determine the maximum value.
  • Period Minimum—this function returns the minimum value from a set of values retrieved from two or more timesteps of the selected variable's time series. This function looks for values in both the Offset and Period fields to determine the range of values to use to determine the minimum value.

Offset 

The offset identifies a timestep in the selected time-series relative to the current timestep. Enter a value in hours.

  • Positive value indicates a future timestep
  • Negative value indicates a past (or completed) timestep

ResSim will convert the offset from hours to timesteps by dividing by the length of the timestep (compute interval) and truncating any remainder. For example, to get the value computed 3 hours earlier, enter a -3 in the offset field.

NOTE: If the compute interval is 2 hours, a -3 offset will cause ResSim to return the value from the previous timestep (an offset of -1 timesteps).

Period

The period is the number of hours over which one of the Period functions computes a value. Enter a value in hours.

  • The period is always back in time relative to the time indicated by the offset.
  • If the offset is zero or blank, the period is back in time relative to the current timestep.

ResSim will convert the period from hours to timesteps by dividing by the length of the timestep (compute interval) and truncating any remainder.

Complete Release Function

When you have identified the independent variable and its Time Series Options, click OK to complete the definition of the Independent Variable of your release function and close the Independent Variable Definition Editor.